1. Technical Field
The present invention relates to an image coding apparatus and an image coding method. Particularly, the present invention relates to a technology of allowing generated code amounts to correspond to a target code amount given to one picture without performing in-screen feedback control.
2. Description of the Related Art
According to the related art, in a system which transmits a bit stream of a moving image or records the bit stream on recording media, high efficiency coding is performed in order to efficiently use a transmission path or a recording capacity.
In an image coding apparatus for realizing the high efficiency coding, a quantization step is controlled such that a coding bit rate of a bit stream generated in an encoder becomes constant in accordance with a transmission rate of a transmission medium. For example, the image coding apparatus suppresses generated data amounts by increasing a quantization step when an image with a complicated pattern is continuous, and increases the generated data amounts by reducing the quantization step when a simple pattern is continuous. The image coding apparatus prevents overflow or underflow from occurring by controlling the quantization step as described above, thereby ensuring a fixed rate.
Thus, in the image coding apparatus according to the related art as described above, when a complicated image is continuous, the quantization step is increased, so that image quality is degraded. When a simple image is continuous, the quantization step is reduced, so that uniform image quality may not be obtained throughout the image.
In this regard, for example, according to Japanese Patent Registration No. 3358620, an assigned code amount assigned to each GOP (Group Of Pictures) is adjusted according to the ratio of the sum of the difficulty of coding for each GOP and the difficulty of coding regarding a plurality of GOPs. For example, a large code amount is assigned to a GOP including an image with a complicated pattern, and a small code amount is assigned to a GOP including an image with a simple pattern.
Meanwhile, as a method of allowing generated code amounts to correspond to a target code amount given to one picture, for example, there is a method called step 2 of a TM5 (test model 5). This denotes a scheme in which a target code amount is set by uniformly distributing a code amount assigned to a picture to macro blocks (MBs) and feedback control is performed in the picture, so that generated code amounts are allowed to correspond to the target code amount.